Scanning probe devices such as the scanning Probe microscope (“SPM”) or atomic force microscope (“AFM”) can be used to obtain an image or other information indicative of the features of a wide range of materials with molecular and even atomic level resolution. In addition, AFMs and SPMs are capable of measuring forces accurately at the piconewton to micronewton range, in a measurement mode known as a force-distance curve or force curve. As the demand for resolution has increased, requiring the measurement of decreasingly smaller forces free of noise artifacts, the old generations of these devices are made obsolete. A demand for faster results, requiring decreasingly smaller cantilevers, only reinforces this obsolescence. The preferable approach is a new device that addresses the central issue of measuring small forces with minimal noise, while providing an array of modules optimizing the performance of the device when using small cantilevers or when doing specialized applications such as optical techniques for biology, optical techniques for photochemical, photothermal, photovoltaic and other light induced changes to the cantilever or sample, nanoindentation and electrochemistry.
For the sake of convenience, the current description focuses on systems and techniques that may be realized in particular embodiments of scanning probe devices, the SPM or the AFM. Scanning probe devices also include such instruments as 3D molecular force probe instruments, scanning tunneling microscopes (“STMs”), high-resolution profilometers (including mechanical stylus profilometers), surface modification instruments, nanoindenters, chemical/biological sensing probes, instruments for electrical measurements and micro-actuated devices. The systems and techniques described herein may be realized in such other scanning probe devices.
A SPM or AFM is a device which obtains topographical information (and other sample characteristics) while scanning (e.g., rastering) a sharp tip on the end of a probe relative to the surface of the sample. The information and characteristics are obtained by detecting small changes in the deflection or oscillation of the probe (e.g., by detecting changes in amplitude, deflection, phase, frequency, etc.) and using feedback to return the system to a reference state. By scanning the tip relative to the sample, a map of the sample topography or other characteristics may be obtained.
Changes in the deflection or oscillation of the probe are typically detected by an optical lever arrangement whereby an incident light beam is directed onto the side of the probe opposite the tip and a reflected beam from the probe illuminates a position sensitive detector (“PSD”). As the deflection or oscillation of the probe changes, the position of the reflected spot on the PSD also changes, causing a change in the output from the PSD. Changes in the deflection or oscillation of the probe are typically made to trigger a change in the vertical position of the base of the probe relative to the sample (referred to herein as a change in the Z position, where Z is generally orthogonal to the X/Y plane defined by the sample), in order to maintain the deflection or oscillation at a constant pre-set value. It is this feedback that is typically used to generate a SPM or AFM image.
It will be noted that the optical lever arrangement measures probe motion indirectly by measuring the angle of reflection of a light beam from the probe to the PSD. A few SPMs and AFMs, particularly earlier manifestations, have measured the motion of the probe directly through the use of an interferometric detection scheme. This method of measuring the motion of the probe gives the user a direct measurement of probe displacement and velocity.
SPMs or AFMs can be operated in a number of different sample characterization modes, including contact modes where the tip of the probe is in constant contact with the sample surface, and AC modes where the tip makes no contact or only intermittent contact with the surface.
Actuators are commonly used in SPMs and AFMs, for example to raster the probe or to change the position of the base of the probe relative to the sample surface. The purpose of actuators is to provide relative movement between different parts of the SPM or AFM: for example, between the tip of the probe and the sample. For different purposes and different results, it may be useful to actuate the sample or the tip or some combination of both. Sensors are also commonly used in SPMs and AFMs. They are used to detect movement, position, or other attributes of various components of the SPM or AFM, including movement created by actuators.
For the purposes of this specification, unless otherwise indicated, the term “actuator” refers to a broad array of devices that convert input signals into physical motion, including piezo activated flexures; piezo tubes; piezo stacks, blocks, bimorphs and unimorphs; linear motors; electrostrictive actuators; electrostatic motors; capacitive motors; voice coil actuators; and magnetostrictive actuators; and the term “sensor” or “position sensor” refers to a device that converts a physical quantity such as displacement, velocity or acceleration into one or more signals such as an electrical signal, including optical deflection detectors (including those referred to above as a PSD and those described in U.S. Pat. No. 6,612,160, Apparatus and Method for Isolating and Measuring Movement in Metrology Apparatus); capacitive sensors; inductive sensors (including eddy current sensors); differential transformers (such as those described in U.S. Pat. No. 7,038,443 and continuations thereof, Linear Variable Differential Transformers for High Precision Position Measurements; U.S. Pat. No. 8,269,485 and continuations thereof, Linear Variable Differential Transformer with Digital Electronics; and U.S. Pat. No. 8,502,525, and continuations thereof, Integrated Micro-Actuator and Linear Variable Differential Transformers for High Precision Position Measurements, each of which is hereby incorporated by reference in their entirety); variable reluctance; optical interferometry; strain gages; piezo sensors; and magnetostrictive and electrostrictive sensors.
Some current SPM/AFMs can take images up to 100 um2, but are typically used in the 1-10 um2 regime. Such images typically require from four to ten minutes to acquire. Efforts are currently being made to move toward what has been called “video rate” imaging. Typically those who use this term include producing images at the rate of one per second all the way to a true video rate at the rate of 30 per second. Video rate imaging would enable imaging moving samples, imaging ephemeral events and simply completing imaging on a timelier basis. One important means for moving toward video rate imaging is to decrease the mass of the probe, thereby achieving a higher resonant frequency while maintaining a lower spring constant.